Conventional methods for the separation of hydrogen from carbonaceous fuels typically require the steps shown in FIG. 1. These include 1) reaction to a syngas stream, 2) a cleanup step, 3) a shift reaction, and 4) a hydrogen separation step.
The syngas is obtained by reacting a fuel with steam, air, or pure oxygen to create a mixture of hydrogen, carbon monoxide, carbon dioxide, water, and lower hydrocarbons. Particulates and contaminants are removed in subsequent steps. The clean syngas stream is then reacted in a cascade of reactors to form as much hydrogen as possible through the water-gas shift reaction by passing it over a catalyst. More advanced shift reactors attempt to attain equilibrium at reduced temperature, while also performing the entire shift reaction in one single reactor. A subsequent separation step is required to remove the CO2, which, in the conventional process, is typically done by pressure swing adsorption (PSA). The aforementioned PSA process can be energy intensive and is not continuous.
Other example methods of hydrogen separation include diffusion methods that use the difference in diffusion coefficients between gas molecules passing through a material. The material has either a microporosity that allows smaller molecules to diffuse at a higher rate than larger molecules, or the material preferentially dissolves certain atoms or molecules. Fouling of the material, as well as cost and energy intensity, are among the reasons that more advanced separation methods are needed.